1. Field of the Invention
This invention relates to improvements in or relating to a vertical roller mill, and more particularly to a vertical roller mill in which the risk of overload occurring during milling of a substance such as cement clinker or blast furnace slag is diminished and whose performance is stable over a long period of time.
2. Description of the Prior Art
Various vertical roller mills are conventionally known, and an example of one of said conventional vertical roller mills, in which the raw material is ground between a table liner mounted on a milling table for rotation about a vertical axis, and milling rollers mounted for rotation about a roller shaft and disposed circumferentially around the axis of rotation of the milling table and designed to be resiliently pressed with a certain force against said table liner, is disclosed in Japanese Examined Patent Publication No. Hei 2-27017 and is of the type shown in FIG. 1. As described in the above Japanese Examined Patent Publication and as shown in FIG. 1, the conventional vertical roller mill is characterized in that the gap (1) formed between the face of the milling roller (2) and that of the table liner (3) of the milling table (4) has a wedge-like sectional shape which decreases in sectional area as one moves towards the circumference of the milling table such that the outlet gap (5) has the minimum sectional area.
However the wear upon the table liner of the milling table and the milling roller is not uniform along the grinding faces of the table liner and the milling rollers and it is observed that relatively little wear occurs at the very outer portions of the grinding face of the milling rollers (i.e. those portions of the grinding faces adjacent the outer side (8) of the milling roller) and corresponding portions of the table liner and as a result these areas become protruded relative to areas located further towards the axis of rotation of the milling table which are subject to severe wear. These protruded areas prevent the smooth discharge of the milled product to the outside of the milling table with the consequent risk of overload occurring. They also prevent much of the grinding aces of the milling rollers and table liner from coming together sufficiently closely for optimum grinding.
There is the possibility that these protruded areas will break off and that as a consequence the risk of overload will diminish as the milled product is once again able to be discharged smoothly to the outside of the milling table and that the grinding efficiency will increase towards its optimum value as the grinding surfaces are able to come closer together again. However a significant time may lapse before such breakage occurs, if it occurs at all, and during this time the grinding efficiency is reduced. Furthermore when the edges do break significant vibrations are produced as a result and said vibrations are harmful to the mill and in the worst case may cause the mill to stop. For these reasons the formation of these protruded areas is not desirable.
For the conventional vertical roller mill, in order to avoid overload, the countermeasure of reducing the supply of feed material to the mill was adopted. However, this results not only in a further reduction of the milling efficiency but also results in an artificial reduction in the thickness of the layer of grinding material between the milling rollers and the table liner, and this results in an increase in the the level of harmful vibrations.